Does the phrase "to err on the side of ...," specifically "err on the side of caution," indicate that it is wrong to be "on the side of," the object? "Err" makes me think of "error," and is such the word in question in the phrase. Additionally, is this phrase considered an idiom?

A slightly different phrase with a slightly different meaning is, "The lesser of two evils." This much more strongly implies that neither of two available options are "good" but rather that one is more preferable than the other.
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MrHenMar 24 '11 at 18:22

To err is human. Everyone knows that. But to arr is pirate.
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user95503Oct 24 '14 at 12:13

5 Answers
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In the phrase “to err on the side of…”, the meaning of err is closer to that of the Old French errer, which is “to stray”. The image implied is that, if you can't for sure walk on the right line, you might want to err on one particular side rather than the other.

So, just to make sure I answer your actual question: no, “to err on the side of…” does not indicate wrongdoing.

@F'x: I must disagree. The phrase "...to err on the side of..." is roughly equivalent to "...to be on the safe side of...". Thus, IMO in this case "err" is not used as if it was derived from errer; rather, it's used to indicate a preference for a safe position. YMMV.
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Bob JarvisMar 24 '11 at 17:10

2

I think that both meanings are applicable. Both the English word err and Old French errer ultimately derive from Latin errare, which means both to wander/stray or make a mistake.
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YozomiriMar 24 '11 at 17:39

"To err on the side of caution" means that if you are wrong you will at least be wrong in a cautious way. Let's say you have some chicken in your fridge. You're not sure if it's edible or not because you don't recall how long it's been there. Even though you are hungry and want to eat it, you throw it out to avoid any chance of food poisoning. Then your wife/husband/roommate gets home and says "I just bought that chicken today!". Well, you erred in throwing it out, but it was through caution.

Basically it means to act conservatively even if your actions are more conservative than they need to be.

It doesn't actually matter if you erred in the end. It's about potential results: if throwing away the chicken is an error then you're out some money and convenvenience; if it's an error to eat the chicken then you get sick.
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Monica CellioAug 20 '12 at 15:33

@MonicaCellio: If you didn't err in the end then you didn't err at all. But erring on the side of caution means you err where it hurts the least, if it is an error. Throwing out good chicken is an error, but a cautious one. Throwing out bad chicken is simply right. Eating good chicken is right. But if you don't know it's good, you act cautiously, and potentially err.
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Mr. Shiny and New 安宇Aug 20 '12 at 17:12

It is used in a situation where you have to make a choice where the desired outcome is uncertain.

Example: You have to make a decision from data about marketing drug A (95% safe, 70% effective) or drug B (70% safe, 95% effective).

Your goal is to make the most amount of money considering both safety and efficacy. However, in life, nothing is absolutely reliable such as this safety/efficacy data.

Which is more important to make the most money — safety or efficacy? You have to make a decision. Let’s say you decided that safety is more important in making more money so that doctor’s and patients feel more safe about the drug to continue sales. However, this could be a mistake — an error — it may very well be that efficacy is more important in making more money.

By choosing to market drug A based on safety concerns even though it may be an error in terms of revenue, you have just erred on the side of safety.

In other words, let’s say if you had to make a mistake (the choice that leads to making no money) which would you choose?

Keeping in mind that your goal is strictly to make revenue, would you rather choose drug A and make the error of being safe but making no money or choose drug B which is effective but not as safe and make no money?

If you’re not a greedy bastard you probably erred on the side of safety and chose Drug A.

This would be a reasonable answer except for lots of typos, mistakes, and informal writing. If you clean this up a bit it might be up-votable.
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MitchJan 25 '12 at 1:53

This is no longer a bad answer, but it's still incorrect, because it doesn't deal with erring at all. To err on the side of safety* in this situation would be not to market either drug: people will still suffer because of your decision, but you choose the course that keeps people safe (but uncured) rather than the one that cures them (at risk of side-effects)
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TimLymingtonNov 2 '12 at 22:02